To: All Members, M.T.O.C.
From: Government Entomologist
Date: 17th November 1972.

Deterrent Effect of DDT - A Preliminary Report on work at Arohane, San Cristobal

During the course of pre-spraying studies on vector bionomics at Arohane village, East Bauro, San Cristobal, a study of the effect of DDT residual spraying on entry of vector species into a house was carried out.

2. Entry was measured on the basis of man-biting collections carried out on an all-night routine.

3. Two species Anopheles farauti (Af) and An. koliensis (Ak) were caught.

4. Spraying took place on 12/4/72.

5. Using a null hypothesis that there is no change in the ratio entering with time that may not have been due to chance the correlation coefficient, calculated using the method in Swaroop (1966), r = 0.876, which for 9 degrees of freedom is significant at the 1% level (Snedecor, after Fisher), Thus it seems that there is a strong correlation between the time elapsed since spraying and the proportion entering the sprayed house compared with the unsprayed house.

6. Looking at the Table, it would seem, moreover, that the deterrent effect of the DDT drops dramatically at around four months after spraying.

7. This particular study can be criticised in that the presence of a single sprayed house in an otherwise unsprayed village is of course an artificial situation and being repelled from this particular house causes no problem to the mosquito in finding other available hosts.

8. I have examined data on indoor and outdoor man-biting levels obtained at Koli, Guadalcanal. Although this data is not wholly comparable with the Arohane experiment, comparison of the levels before and after the first spraying cycle show a change from dominant indoor biting, 2.31 indoors : 1.00 outdoors, pre-spraying October to April 1963, to dominant outdoor biting, 0.28 indoors : 1.00 outdoors, post-spraying May to December 1963.

9. More recent data from the same area is confused because of the erratic intervals between cyclical spraying, followup spraying and interval spraying. However, the ratio of indoors : outdoors is 0.25:1.00 for the period January to December 1971.

10. The work at Arohane will continue on a somewhat different basis as the whole village with the exception of one house (the unsprayed fixed station in the above series) has now been sprayed, on 11th October, 1972. Bearing in mind that the onset of spraying operations usually has led to a large reduction in overall vector densities, the results of this continuing experiment should be of interest.

11. It possibly is unwise to draw conclusions from these early results but the deterrent effect of DDT obviously is a factor, and probably a major factor, in the interruption of transmission of malaria. The behaviour of the human population can equally offset this deterrent effect and this has been repeatedly commented on by the various entomologists associated with this programme viz Mataika, in Quarterly Report, October-December 1963; Sloof, in Assignment Report, January 1969; myself, in paper to Inter-Territorial Conference at Sydney, October 1971; and van Seventer, in Assignment Report, April 1972.

Brian Taylor
Government Entomologist.

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